冠词“类别”的区别
a/an/the+单数可数名词 这是可以互换吗?但又有什么差别?例：what's this in English?----It's (a) ruler 这句话可以用the+单数可数名词吗?It's said that (the) umbrella was invented over four thousand years ago ba Chinese people.这句话可以用a/an+单数可数名词吗?